Gas monitors and gas detection devices are well-known, having been developed as analytical tools and for use in monitoring commercial operations where it is necessary to detect the presence of a particular gas. Gas detection devices are now in increasing demand to monitor the quality of air in the workplace, especially to detect the presence of high levels of a toxic or combustible gas. Due to the varied properties of gases to be monitored and the widespread conditions under which such devices are needed, many devices have been developed for monitoring and detecting gases, each which suits the particular circumstances and environment where the device is to be used.
These devices operate on diverse technologies such as optical, chemical, gas chromatographic, electrochemical, and catalytic detection. Optical devices rely on detection of the infrared absorption spectra of the gas to be monitored. Such systems are expensive and not portable. Continuous monitoring and immediate feed back is not feasible with such a device. Devices which rely on chemical reactions, such as calorometric systems, are slow, non-continuous and costly. A device which utilizes gas chromatography to detect the presence of a particular gas also has inherently slow response time and cannot provide a continuous monitoring of the environment to detect the indicated gas. Electrochemical gas detection generally occurs in a cell wherein an oxidation-reduction reaction occurs with the gas to be detected dissolved in an electrolyte. This type of system is generally not highly selective for the particular gas to be detected. Devices which utilize catalytic sensors generally rely on a thermotype sensor which measures the temperature change of the catalytic material or a semiconductor catalytic sensor. Well-known semiconductor type sensors used in alarm devices and analyzers to date include zirconium oxide, titanium oxide, indium oxide, tin oxide, tungsten oxide, platinum and palladium doped metal oxides, and mixtures thereof. These semiconductor materials generally monitor the impedance changes in the oxide when a catalytic reaction takes place. The semiconductor material is generally heated to provide a controlled reaction environment. Hooker recognized that semiconductor catalytic sensors are sensitive to humidity and provided a moisturizing ring to surround the sensor in his U.S. Pat. No. 3,933,433 entitled, "Method and Apparatus for Gas Detection".
Recently, silicon based devices have been reported which have sensitivities to particular gases. Lundstrom et al. reported in Applied Physics Letters, Vol. 26, No. 2, Jan. 15, 1975, a hydrogen-sensitive MOS field effect transistor which comprised an MOS field effect transistor having a thin palladium film as the gate electrode. Such sensors however, have not been widely commercialized to date.
A major drawback to the commercialized technologies for detecting gas concentrations is the bulk size and power requirements of the detection device. Although permanently mounted fixed monitors may be placed in high risk areas, there is also a great demand to provide portable, light-weight and reliable devices that can continuously monitor and detect significant concentrations of desired gases. Such a monitor may be worn by an individual and continuously monitor the atmospheres to which the individual is exposed as the individual moves throughout the workplace.
Co-pending patent application, U.S. Ser. No. 811,548, entitled "Method and Device for Detection of Changes in Gas Concentration", discloses a light weight, low-power apparatus for continuously monitoring the concentration of at least one selected gas and means for signaling when the gas concentration suddenly increases. The apparatus described therein comprises at least one semiconductor sensor that is sensitive to at least one selected gas and electronic circuitry for monitoring the current changes produced in the semiconductor device. The electronic circuitry produces a voltage change in response to the current output from the semiconductor sensor which is monitored through integrating operational amplifiers. The amplifiers have differing time constants so that voltage changes produce unequal voltage responses. When the voltage responses deviate in a manner indicating sudden gas concentration changes an alarm is activated. While this apparatus is a significant improvement to the field of personal safety alarms, it, like previous gas detection devices, may not respond similarly under various conditions of temperature and humidity. This is a drawback to a portable gas detection device relied upon by an individual who may travel through localized areas having widely differing temperature and humidity levels.
It would be a significant contribution to the field of portable gas detection devices to provide a gas detection and alarm device that is miniature, light-weight, low-power and relatively insensitive to at least one of temperature and humidity fluctuations.
Thus, one object of the present invention is to provide a gas detection and alarm device that is not affected by variations in at least one of temperature and humidity.
It is another object of the present invention to provide a method for the detection of changes in gas concentrations, that is relatively insensitive to at least one of temperature and humidity fluctuations.
These and other objects of the present invention will become obvious to one skilled in the art from the below description of the invention and the appended claims.